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Subject[RFC] Standard way of generating assembler offsets
Date
Almost every architecture generates Assembler values to map the offsets
of fields in C structures, about the only exception is i386 and that is
because its offsets are hard coded into entry.S. Every arch has done
it differently, none of them have got it exactly right.

As part of kbuild 2.5 I am standardizing on one method for generating
Assembler offsets. This change is required for kbuild 2.5 but it can
be added to 2.4 without disturbing the current kbuild, I want to do
this gradually now instead of a single massive change in kernel 2.5. I
will be issuing per architecture changes for generating Assembler
offsets against 2.4.

The kbuild 2.5 method for generating Assembler offsets satisfies these
requirements:

* No manual intervention required. Many architectures rely on users
running make dep after changing config options that affect the
Assembler offsets. If the user forgets to run make dep then the C
and Assembler code is out of sync - totally unacceptable. This is
completely fixed in kbuild 2.5; I cannot do a complete fix in kbuild
2.4 but it is still better than the existing manual system.

* Standard name for the related files. There are 6+ different names
for the files used to generate Assembler offsets, kbuild 2.5 uses
asm-offsets.[csh] on all architectures.

* Allows for multiple parallel compiles from the same source tree.
Writing the generated asm-offsets.h to include/asm is not an option,
it prevents concurrent compiles.

* The method must work in native and cross compile mode and give
exactly the same results. Some 2.4 code only works in native mode,
some architectures have different methods for native and cross
compile with different output formats. Yeuch!

* Standard scripts for generating the output. Every arch does it
differently in 2.4, standards are good!

* Correct dependency trees. Because 2.4 make dep does not scan .S
files, there is little or no dependency information. Even if the
offsets are regenerated, the affected Assembler code does not always
get rebuilt. kbuild 2.5 handles dependencies for Assembler as well
as C; I cannot get kbuild 2.4 perfect but I can improve on the
existing (or non-existent) 2.4 dependencies.

All architectures will define arch/$(ARCH)/asm-offsets.c. This has a
standard prologue for the macros that convert offsets to Assembler,
followed by arch specific field references.

arch/$(ARCH)/asm-offsets.s is generated from arch/$(ARCH)/asm-offsets.c
using standard rules, although kbuild 2.4 needs some tweaking.

arch/$(ARCH)/asm-offsets.h is generated from arch/$(ARCH)/asm-offsets.s
by a semi-standard script. Most of the script is common to all
architectures but the precise format of the Assembler output is arch
specific.

The final result is included in *only* the Assembler programs that need
it, as #include "asm-offsets.h" with -I arch/$(ARCH) in the relevant
Makefiles. Hard coding relative paths in source files is a pet hate,
use #include "localname.h" and -I instead. Including the generated
file in C code is not allowed, it severly pollutes the dependency
chain, to the extent that any config change can force a complete
recompile, unacceptable.


Example from i386:

arch/i386/asm-offsets.c

/*
* Generate definitions needed by assembly language modules.
* This code generates raw asm output which is post-processed to extract
* and format the required data.
*/

#include <linux/types.h>
#include <linux/stddef.h>
#include <linux/sched.h>

/* Use marker if you need to separate the values later */

#define DEFINE(sym, val, marker) \
asm volatile("\n-> " #sym " %0 " #val " " #marker : : "i" (val))

#define BLANK() asm volatile("\n->" : : )

int
main(void)
{
DEFINE(state, offsetof(struct task_struct, state),);
DEFINE(flags, offsetof(struct task_struct, flags),);
DEFINE(sigpending, offsetof(struct task_struct, sigpending),);
DEFINE(addr_limit, offsetof(struct task_struct, addr_limit),);
DEFINE(exec_domain, offsetof(struct task_struct, exec_domain),);
DEFINE(need_resched, offsetof(struct task_struct, need_resched),);
DEFINE(tsk_ptrace, offsetof(struct task_struct, ptrace),);
DEFINE(processor, offsetof(struct task_struct, processor),);
BLANK();
DEFINE(ENOSYS, ENOSYS,);
return 0;
}

asm-offsets.s to asm-offsets.h.

# Convert raw asm offsets into something that can be included as
# assembler definitions. It converts
# -> symbol $value source
# into
# symbol = value /* 0xvalue source */

echo '#ifndef __ASM_OFFSETS_H__'
echo '#define __ASM_OFFSETS_H__'
echo '/*'
echo ' * DO NOT MODIFY'
echo ' *'
echo " * This file was generated by arch/$(ARCH)/Makefile.in."
echo ' *'
echo ' */'
echo ''
awk '
/^->$/{printf("\n")}
/^-> /{
sym = $2;
val = $3;
sub(/^\$/, "", val);
$1 = "";
$2 = "";
$3 = "";
printf("%-20s = %3d\t/* 0x%x\t%s */\n", sym, val, val, $0)
}
'
echo '#endif'

Generated arch/i386/asm-offsets.h

#ifndef __ASM_OFFSETS_H__
#define __ASM_OFFSETS_H__
/*
* DO NOT MODIFY
*
* This file was generated by arch/i386/Makefile.in.
*
*/

state = 0 /* 0x0 offsetof(struct task_struct, state) */
flags = 4 /* 0x4 offsetof(struct task_struct, flags) */
sigpending = 8 /* 0x8 offsetof(struct task_struct, sigpending) */
addr_limit = 12 /* 0xc offsetof(struct task_struct, addr_limit) */
exec_domain = 16 /* 0x10 offsetof(struct task_struct, exec_domain) */
need_resched = 20 /* 0x14 offsetof(struct task_struct, need_resched) */
tsk_ptrace = 24 /* 0x18 offsetof(struct task_struct, ptrace) */
processor = 52 /* 0x34 offsetof(struct task_struct, processor) */

ENOSYS = 38 /* 0x26 ENOSYS */
#endif


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